Significant growth in both high-frequency wired and wireless markets has introduced new opportunities where compound semiconductors such as SiGe have unique advantages over bulk complementary metal oxide semiconductor (CMOS) technology. With the rapid advancement of epitaxial-layer pseudomorphic SiGe deposition processes, epitaxial-base SiGe heterojunction bipolar transistors have been integrated with mainstream CMOS development for wide market acceptance, providing the advantages of SiGe technology for analog and RF circuitry while maintaining the full utilization of the advanced CMOS technology base for digital logic circuitry.
It is known to incorporate impurities into the various regions of SiGe heterojunction devices in order to improve device performance. For example, the emitter region is typically doped with arsenic (As) and the base region is typically doped with boron (B) in order to improve the ac performance (i.e. fT, fmax) of the SiGe bipolar transistor.
The incorporation of carbon (C) into SiGe heterojunction devices has been carried out in the prior art to prevent the out-diffusion of boron from the base region. For example, it is known that the transient enhanced diffusion of boron is strongly suppressed in carbon-rich silicon layers; see, for example, H. J. Osten, et al., “Carbon Doped SiGe Heterojunction Bipolar Transistors for High Frequency Applications”, IEEEBTCM 7.1, 109. Boron diffusion in silicon occurs via an interstitial mechanism and is proportional to the concentration of silicon self-interstitials. Diffusion of carbon out of the carbon-rich regions causes an under saturation of silicon self-interstitials. As a result, the diffusion of boron in these regions will be suppressed.
Efforts to further improve the ac performance of SiGe heterojunction devices include increasing impurity concentrations such as As in the emitter region and C in the base region. However, it has been observed that although ac performance improves with increasing As and C concentrations, the noise performance and reliability of SiGe heterojunction devices are degraded. The degradation in noise performance and reliability is due in part to the higher impurity concentrations resulting in an increased number of carriers near the emitter-base junction region.
In view of the SiGe bipolar performance problem mentioned above, there is a continued need for improving noise performance and reliability in SiGe bipolar devices having high impurity concentrations in the emitter and base regions.